A flange may be a cast iron (gray iron or ductile iron) plate of nominal size, generally annular in shape, designed to connect lengths of centrifugally cast gray-iron or ductile-iron pipe of corresponding nominal size. A flange of a given nominal size (generally expressed in inches for North American flanges and pipes) may also have a nominal outer diameter. A length of cast iron pipe of a given nominal size may have a nominal outer diameter, a minimum nominal thickness depending on its rated water pressure, and a nominal allowance for casting tolerance. A flange may be incorporated into a length of cast iron pipe. The flange may also be a separate annular plate coupled to a length of pipe via threading. Taper pipe threads on the internal surface of the flange correspond to threads on the external surface of the pipe. Two lengths of pipe may be connected by bolting or screwing the corresponding flanges to each other, face to face, through a plurality of bolt holes evenly distributed in a concentric circle around the perimeter of the flange. The exact size and number of bolt holes will depend on the nominal size of the flange. A gasket positioned between the two flanges, also annular in shape and fashioned from rubber or a like material, may seal the connection.
“Corresponding pipe” and “corresponding flange” refer to the fact that ductile iron flanges of nominal size are standardized to fit ductile iron pipe of the same nominal size. For example, ANSI/AWWA C115/A21.15-11, which is incorporated by reference herein in its entirety, provides that North American ductile iron pipe of 8-inch nominal size may correspond to a standard flange of 8-inch nominal size having an outer diameter of 13.5 inches and eight evenly spaced circular bolt holes of ⅞-inch diameter.
On a large scale construction project, which may involve multiple flanged connections as pipes are run into buildings, through walls, etc., “two-holing” is standard practice to ensure that any two flanges or flanged pipe spools of a given nominal size are properly aligned. A flange is properly two-holed when a horizontal centerline connecting the top two bolt holes lies perpendicular to the vertical centerline of the pipe. Section 4.4.4 of ANSI/AWWA C115/A21.15-11 provides that misalignment of corresponding bolt holes of the two flanges should not exceed 0.12 inch measured from like points on bolt holes. Similarly, flange faces should be perpendicular to the pipe centerline and parallel, such that face-to-face dimensions at opposite points on the outer diameter of the flanges do not differ by more than 0.06 inch. When flanges are properly aligned, any connecting valves, hydrants, fittings, etc. should fit uniformly.
Problems arise when flanges and pipe spools are improperly two-holed or misaligned, which may happen for a variety of reasons including but not limited to misaligned plant walls, improperly poured concrete floors, inaccurate equipment specifications, or simple human error. In response to an improperly aligned flange, a builder may tear down and rebuild a wall, pull up and re-pour a floor, or order a new pipe spool piece. Any of these courses of action may lead to delayed completion, increased expenses, or both. Where an improperly two-holed flange may be only slightly misaligned with its companion flange, however, the builder may force a connection of the two flanges, bolting them together at an improper angle. A forced connection may appear to resolve the problem of two misaligned flanges but only conceals and complicates the problem, leading to increased potential for leaking (and further flange alignment issues) at multiple pipe joints. Leaks may only be discovered long after the building has been completed and the builders have left. Equipment suppliers, rather than pipefitters, may be called to account for these leaks. While several solutions to the problem of improper two-holing have been developed, these solutions may present new challenges.